Intrusion Detection System for Use on Single Mode Optical Fiber Using a Simplified Polarimeter

ABSTRACT

A telecommunications optical fiber is secured against intrusion by detecting manipulation of the optical fiber prior to an intrusion event. This can be used in a non-locating system where the detection end is opposite the transmit end or in a locating system which uses Fresnel reflections and Rayleigh backscattering to the transmit end to detect and then locate the motion. The Rayleigh backscattering time sliced data can be stored in a register until an intrusion event is detected. The detection is carried out by a polarization detection system which includes an optical splitter which is manufactured in simplified form for economic construction. This uses a non-calibrated splitter and less than all four of the Stokes parameters. It can use a polarimeter type function limited to linear and circular polarization or two linear polarizers at 90 degrees.

RELATED APPLICATIONS

This application is a divisional application from application Ser. No.11/152,680 filed Jun. 15, 2005 and now issued as patent no INSERT WHENKNOWN.

This application claims priority under 35 U.S.C. 119 from ProvisionalApplications 60/643,001, 60/643,002, 60/643,003 and 60/643,004 all filedJan. 12, 2005.

Reference is made to co-pending applications filed on the same date asthis application by the same inventors, the disclosures of which areincorporated herein by reference as follows:

Application Ser. No. 11/152,679 attorney docket number 85570-302entitled AN INTRUSION DETECTION SYSTEM FOR USE ON SINGLE MODE OPTICALFIBER USING FRESNEL REFLECTIONS.

Application Ser. No. 11/152,772 attorney docket number 85570-332entitled AN INTRUSION DETECTION SYSTEM FOR USE ON SINGLE MODE OPTICALFIBER USING A STORAGE REGISTER FOR DATA.

Application Ser. No. 11/152,681 attorney docket number 85570-202entitled AN INTRUSION DETECTION SYSTEM FOR USE ON SINGLE MODE OPTICALFIBER USING A POLARIMETER.

Application Ser. No. 11/152,663 attorney docket number 85570-242entitled AN INTRUSION DETECTION SYSTEM FOR USE ON SINGLE MODE OPTICALFIBER USING A COST REDUCED POLARIMETER.

Application Ser. No. 11/152,768 attorney docket number 85570-402entitled AN INTRUSION DETECTION SYSTEM FOR USE ON SINGLE MODE OPTICALFIBER USING A POLARIZERS AT 90 DEGREES.

FIELD OF THE INVENTION

The present invention relates to the detection of movement of a singlemode optical fiber.

The present invention as described in more detail hereinafter includesboth to embodiments of the invention which allow location of theintrusion event by analysis at the same end as the transmitted light ofreflected light and to embodiments of the invention which do not providelocation of the intrusion event by analysis of light at a remote end ofthe fiber.

BACKGROUND OF THE INVENTION

Single mode fiber optic backbone cables are being deployed to connectsections of high-speed networks together, and for long distancecommunications. To secure these high-speed networks, software basedIntrusion Detection Systems (IDSs) have been introduced. These systemscapture and analyze all packets for unusual patterns that point to anintrusion as well as monitor systems accessing a network. However, thisadds to the complexity of the network and burdens processing power.Current IDSs are hampered by Base-Rate Fallacy limitation, which is theinability to suppress false alarms. Additionally, software-based IDSs donot provide protection against passive optical fiber tapping, which cango undetected by the network hardware. Software IDS is the de-factostandard for intrusion detection, however it is oblivious to actualphysical layer intrusion and perturbation such as tapping or theattendant fiber handling.

It is well known, by those skilled in the technology, that opticalfibers are easily tapped and the data stream monitored. One relativelysimple non-interruptive tapping method involves placing a bend coupleron the fiber to be tapped. A controlled bend of a critical radius isplaced on the fiber. This causes a small spatial distortion in thecore/cladding guiding properties and a fraction of the light escapes thefiber. A detector is located at the point of the light leakage and thedata steam observed. Bend couplers typically introduce a loss of lightpower of up to 1 dB or more. Power measuring intrusion detection systemsare available to detect this loss in optical power and provide warningalarms.

With care and skill, more insidious methods are available to the skilledintruder. With a sufficiently sensitive receiver and care inpreparation, a fiber can be successfully tapped without introducing atelltale bend in the optical fiber. A successful tap can be achieved bycarefully removing a few inches of the protective outer coating of thetarget fiber and polishing, etching, or otherwise reducing the outercladding down by a few microns to form a flat coupling region. Acladding-to-cladding coupling is then made using a special interceptfiber. This method intercepts a portion of the weak but measurableevanescent power that propagates along the tapped fiber. In this case,the intercepted light, which is detected by a sensitive receiver, caneasily be 20 or 30 dB down from the power in the fiber core. Thisresults in a loss of received optical power of only 0.04 or 0.004 dB andis impossible to detect reliably by power measurement methods. Using asimilar stripping mechanism and a high sensitivity photo detector,Rayleigh Scattering from within the fiber can be detected.

Reference is made to Hernday, P. Polarization Measurements. In D.Derickson (Ed.), (1998). Fiber Optic Test and Measurement (pp. 220-245).New Jersey: Prentice Hall PTR, the disclosure of which is incorporatedherein by to reference.

Reference is also made to US pending Application 2005/0002017 publishedJan. 6, 2005 by Haran which discloses primarily a method for utilizingan optical fiber for use in detection of intrusion through a perimeterfence but also mentions in passing that similar techniques can be usedin optical fibers in transmission systems.

Reference is also made to PCT pending Application WO 02/095349 publishedNov. 28, 2002 by Rogers which discloses a method for optical fiberbackscatter polarimetry.

In U.S. Pat. No. 5,384,635 (Cohen) published Jan. 24, 1995 is discloseda method for detecting vibration of an optical fiber caused by diggingequipment where the method detects polarization changes inback-scattered light.

In U.S. Pat. No. 4,904,863 (McDearmon) published Feb. 27, 1990 isdisclosed a pressure sensor which uses an optical fiber where the methoddetects polarization changes in back-scattered light.

In U.S. Pat. No. 4,840,481 (Spillman) published Jun. 20, 1989 isdisclosed a strain sensor which uses an optical fiber where the methoddetects polarization changes in back-scattered light.

In U.S. Pat. No. 6,724,469 (LeBlanc) published Apr. 20, 2004 isdisclosed a method of polarization optical time-domain reflectometry.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide an arrangement fordetecting movement of an optical fiber which overcomes the limitationswith power loss detection methods and can detect intrusion activitybefore any optical power loss occurs.

According to the invention there is provided a method for detectingmovement of an optical fiber comprising:

providing a optical fiber having a first end and a second end;

detecting movement of a portion of the fiber along the length thereofby:

injecting polarized light into one end of the optical fiber;

detecting at one end of the fiber a series of received light signalswhich have been transmitted along the fiber;

comparing at least some of the received light signals relative to dataobtained from previously received ones of the received light signals todetect changes of polarization of the received light signals relative tothe previously received light signals;

analyzing the changes in polarization to determine any changes which areindicative of manipulation of the optical fiber causing movement of aportion thereof along the length thereof;

and generating an alarm in response to the detection of any such changeswhich are indicative of manipulation of the optical fiber causingmovement of a portion thereof along the length thereof;

wherein the changes in polarization are detected by splitting thereceived light signals into no more than three paths including first andsecond paths and using the first path to detect an amplitude of thelight in the path when polarized in a linear direction and using thesecond path to detect an amplitude of the light in the path whenpolarized in a circular direction.

Preferably the changes in polarization are detected by splitting thereceived light signals into only first and second and third paths andusing the first path to detect an amplitude of the light in the pathwhen linearly polarized and using the second path to detect an amplitudeof the light in the path when circularly polarized and using the thirdpath to detect an amplitude of the light in the path when un-polarized.

Preferably the changes in polarization are detected by splitting thereceived light signals into only first and second paths and using thefirst path to detect an amplitude of the light in the path when linearlypolarized and using the second path to detect an amplitude of the lightin the path when circularly polarized.

Preferably the circularly polarized light in the second path is isolatedby a quarter wave retarder and a linear polarizer.

Preferably the light in the second path is circularly polarized by aquarter wave retarder and a linear polarizer.

Preferably the light signals are split into the separate paths by asplitter.

Preferably the splitter is selected such that the state of polarization(SOP) of the signals in the separate paths are NOT maintained relativeto an absolute reference as would be required in a standard polarimeter.

Preferably the light signals are split into the separate paths by anoptical switch which separates the signals in time division such thatthe paths are selected sequentially.

Preferably there is provided a second optical switch or a coupler forsupplying the signals from the separate paths to a single receivingsystem for analyzing the amplitude.

Preferably the light in the second path is circularly polarized by aquarter wave retarder and a linear polarizer and wherein the second pathis fed back to the optical switch so as to use a single linear polarizerfor both the first and second paths.

Preferably an absolute value is obtained of the change in amplitude ineach path between a signal and previous signals and the absolute valuesare summed together to provide an output for analysis.

Preferably an absolute value is obtained of the change in amplitude ineach path between a signal and previous signals and the absolute valuesare summed together to provide an output for analysis.

Preferably an absolute value is obtained of the change in amplitude ineach path between a signal and previous signals and the absolute valuesare summed together to provide an output for analysis.

Preferably the signals are detected at the opposite end of the opticalfiber from which the light is injected.

Preferably the method includes determining the location along the fiberof the said manipulation by:

detecting the signals at the same end as the light pulses are injectedsuch that the signals contain reflected and/or Rayleigh backscatteredcomponents;

detecting polarization of a series of the light signals from theRayleigh backscattering components in discrete time steps to generatefor each time step data relating to the polarization;

such that the stored data is time dependent and thus indicative of atime of travel of the light signals and thus of a location of theposition from which the Rayleigh backscattering components haveoriginated along the fiber;

storing the data in a register for a period of time and discarding thedata after the period of time and replacing it with fresh data;

and, in the event that movement is detected of the optical fiber,extracting the data from the register and analyzing the polarization ofthe series of signals to detect the location of the movement.

Preferably the register is a FIFO.

Preferably the scattering signal level is typically orders of magnitudelower than the Fresnel Reflections and the Fresnel Reflections aretypically infrequent and wherein the reflections are integrated alongwith the scattering such that the Fresnel Reflections integrate into amanageable signal and the total integrated signal is monitored forindication of fiber manipulation.

Preferably the scattering signal level is typically orders of magnitudelower than the Fresnel Reflections and the Fresnel Reflections aretypically infrequent and wherein the large Fresnel reflections aresampled using a storage technique, this stored sample is compared toother dynamic or stored samples and this comparison is monitored forindication of fiber manipulation.

Thus there is provided an intrusion detection system that can sense andalarm any attempt to access the optical fibers in a single mode fiberoptic communication cable. The present method monitors the active signalof a single mode optical fiber strand for signal degradation anddisturbances in polarization that could indicate fiber damage, handling,or physical intrusion.

The system uses the polarized light output signal from a light sourcesuch as, but not limited to, a laser transmitter that is coupled to thesingle mode fiber; standard semiconductor lasers such as DFB andFabry-Perot are inherently highly polarized. At the receive end of thelink, a detection system determines the state of polarization (SOP) ofthe light. Mechanical disturbances such as handling of the fiber cablecause shifts in the SOP that is detected by the system and signals apossible intrusion attempt before an actual tap occurs.

Using adaptive filtering, normal background disturbances fromenvironmental heating/cooling systems, road traffic, and backgrounddisturbances can be learned and filtered out. This will allow maximumsensitivity to intrusion attempt signatures while minimizing theprobability of false alarm events. The design objective is to identifyintrusion attempts while the attack is still at the outer layer of thecable structure. This will allow for rapid location and interception ofany intruder.

Further claimed is the detection of fiber handling and/or intrusion bymethod of monitoring state of polarization, degree of polarization, orof other parameters related to SOP and DOP. This includes detection ormeasurement of the handling or disturbance of the optical fiber orcable, either as a prelude to, incident of, or as a result of anintrusion, as detected by any shift in the degree or state ofpolarization of any portion of the light contained herein, originatingfrom, or propagating through the optical fiber or cable being monitored.

Further claimed is a means for directing the optical transmission ofinformation into any of a plurality of optical fibers. This could be,but is not limited to an optical switch. Significant to this embodimentis the monitoring of all secondary fibers for intrusion, such as withthis invention. The intention is to maintain the security and integrityof all possible fibers from intrusion in order to prevent a pre-emptiveintrusion prior to the re-routing of data.

For illustration, if a perpetrator had unmonitored access to thesecondary fiber, a fiber tap could be installed undetected. The primaryfiber could then be perturbed, and when data is switched to thesecondary, the data security is compromised. According to thisinvention, when an intrusion is attempted on any fiber, it will bedetected; guaranteeing for the future the security of the system.

This summary describes a number of embodiments of this invention:

An embodiment which utilizes only the signals corresponding to S0 andS1, or any other configuration using fewer than the 4 signals requiredfor a full polarimeter. This trade off enables benefits including, butnot limited to, a simplified manufacturing process in exchange forimpact including, but not limited to, decreased sensitivity.

A secondary embodiment on the above theme utilizing a second opticalcoupler summing the intrusion and attenuation signals together, by aconfigurable mix amount. This allows a single optical receiver/detector.The obvious financial advantage of this implementation must be weighedagainst the extreme sensitivity to interference between the two coherentlight paths being combined.

A third embodiment which utilizes a single linear and a single circularpolarization sensitive detectors. This does not yield absolute SOPmeasurements, but will indicate a change in the SOP sufficient forintrusion detection.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, a first set of embodiments is disclosed inwhich the detector is at the remote end of the fiber for detection ofthe intrusion events without locating the events in position along thefiber:

FIG. 1 illustrates a polarized light source launched into a length ofsingle mode fiber. The single mode fiber is connected to an opticalpolarimeter, which feeds a processor.

FIG. 2 is a variation on FIG. 1 above with the following distinctions:the polarimeter block is detailed as a full polarimeter with fourreceivers detecting signals for a Jones Matrix of Stokes Parameters, andthe addition of a 2:1 fiber optic coupler and an additional receiver formonitoring the non-intrusion signal.

FIG. 3 is a block diagram of a simplified polarimeter intrusiondetection system. The output of the individual polarimeter sections ofthe optics are summed in order to detect changes rather than absolutepolarization measurements.

FIG. 4 is a block diagram of a Greatly Simplified SM IDS using a singlepolarizer to detect change in SOP.

FIG. 5 is a block diagram of a Single Receiver SM IDS uses a secondarycoupler to rejoin the multiple light paths into one detector/receiver.

FIG. 6 is a block diagram of a Simplified Polarimeter using one linearpolarizer for detection of linearly polarized light, and one ¼ waveretarder feeding a linear polarizer for detection of circularlypolarized light.

FIG. 7 is a block diagram of a further Simplified Polarimeter.

In the following drawings, a second set of embodiments is disclosed inwhich the detector is at the transmit end of the fiber for detection ofthe intrusion events while locating the events in position along thefiber by using reflected and backscattered light.

FIG. 8 illustrates a block diagram of a polarimeter used on an opticalfiber to detect and locate manipulation of the fiber leading to apotential intrusion event.

FIG. 9 is a block diagram of the Polarization Receiver from FIG. 8.

FIG. 10 is a block diagram of a preferred embodiment of detection systemusing a simplified polarimeter for detection of intrusion events and forlocating those intrusion events along the fiber.

FIG. 11 is a block diagram of an alternative embodiment to that of FIG.10.

FIGS. 12, 13 and 14 show block diagrams of arrangements in which awavelength selective reflection can be used, such as a printed BraggGrating, a reflective connector, or a fiber loop back, as shownrespectively for increasing or controlling the level of the Fresnelreflections from the remote end.

In the following drawings, a third set of embodiments is disclosed inwhich the detector is at the remote end of the fiber for detection ofthe intrusion events utilizing two polarizers arranged at 90 degrees:

FIG. 15 illustrates a polarized light source launched into a length ofsingle mode fiber. The single mode fiber is connected to an opticalpolarizer, which feeds an optical receiver.

FIG. 16 is a block diagram for an installation in an active fiber whichto is being used for data. This is similar to FIG. 1 above, with theaddition of a fiber optic coupler samples a portion of the signal foranalysis, routing the remaining signal to the end user's receiver.

FIG. 17 is similar to FIG. 1 above, with the addition of a 2:1 fiberoptic coupler and a second receiver for monitoring the non-intrusionsignal.

FIG. 18 is a block diagram for a combination of FIGS. 16 and 17 above,the coupler for feeding the end-user equipment, and a dual receiverconfiguration.

FIG. 19 is a block diagram for a variation on FIG. 1 with thesubstitution of a rotatable polarizer under a closed feedback loop.

FIG. 20 is a block diagram of one embodiment of the invention.

DETAILED DESCRIPTION

Fundamental to the present invention is the mechanics or more simply bylaunching a light source of stable polarization 1 into a single modefiber 2. At the remote or receive end the single mode fiber is connectedto the input of an optical polarimeter 3. This polarimeter measures theSOP of the monitored light. The output of this polarimeter is connectedto a processor unit 4; such as, but not limited to, a microcomputer.Handling of the fiber cable causes a local mechanical disturbance to thefiber. This mechanical disturbance, while not introducing detectablemacro or micro bending losses, causes the polarization orientation tochange. This is detected by the polarimeter and reported to theprocessor.

A more comprehensive view is now described in conjunction with FIG. 2.Optical signal feeds the polarimeter 9, which converts it to the fourso-called Stokes Parameters: S0, S1, S2, and S3 as detected by receiversRx0-Rx3 10, 11, 12, and 13. These parameters collectively describe allpossible states of polarization and degrees of polarization (DOP)(Hernday 1998). This is forwarded to the processor 14 where the SOP andDOP are calculated, and the signal is filtered to eliminate normalenvironmental background noise. The filtered signal is then analyzed fortransient signatures and level changes that are characteristic of cableand fiber handling. At a pre-set detection condition the circuitactivates the alarm response.

The optical signal can be split by an optional optical coupler 7. Themain portion of the signal can be brought back out to an opticalconnector (8) and be made available for the communication or datareceiver, sending a sample to the polarimeter for monitoring.Additionally, the S0 parameter of the polarimeter directly measuresoptical power without polarization effects. This can be used to monitorthe received power for perturbations which are detectable in thenon-polarization domain.

A simplified embodiment is depicted in FIG. 3. In an intrusion detectionsystem, actual SOP is not required as information, only the change inthat state. Such an instrument as shown.

Rather than use extensive DSP to calculate DOP, orientation, and angle,a system is presented here for pragmatically determining polarizationchanges only.

The outputs from a polarimeter 15 are individually monitored in additionto being sent to processors such 20, 21, 22 as differentiators andprecision full wave rectifiers (absolute value circuits). These threerectified lines are then summed in a summing amp 19, the output of whichis monitored.

Whenever any change in polarization properties occurs, the distributionof power between the polarimeter outputs (which we will refer to as s1,s2, and s3 16, 17, 18) which lead to the calculation of StokesParameters S1, S2, and S3 changes; as one increases, it is at theexpense of one or both of the others decreasing. If all three lines aredifferentiated and full wave rectified by the processors 20, 21, 22, theincrease on one line will appear as a positive signal into the summingamp, as will the decreases on the other two lines. This is furtherillustrated in FIG. 7. This additive action increases sensitivity of thesystem, always positive going and of level representative of thedisturbance of the intrusion. Also, by monitoring both S0 and the threenon-processed lines s1, s2, and s3, disturbances not related tointrusion, such as a shift in laser power, could be recognized andprocessed. One possible way to analyze that condition would be, when theS0 changes, to confirm that the ratio of the s1-s3 remains the same (itspolarization “signature”) and confirm that total power changes the samedegree as S0.

Alternatively, processors 20, 21, 22 can convert the s parameters toStokes Parameters S. Summing the three processed S parameters of S1, S2,and S3 and monitoring the summed level gives an indication ofpolarization shifting indicating a possible intrusion.

The differentiation and precision rectification can be performed insoftware if desired.

The embodiment in FIG. 4 is for a limited implementation of apolarimeter. This device will detect light that is fully or partiallylinear polarized. Ease of manufacture and low cost are among theadvantages, inability to detect light with circular polarization is adisadvantage. As before, light is injected by a polarized light source23 into the fiber under test 24. This delivers light into a splitter 25that directs a portion of the light unaltered to a detector/receiver 26,and the other portion passes through a polarization filter such as alinear polarizer 27 and into a receiver/detector 28. The advantage ofthis embodiment is that a mixture of polarization sensitive andinsensitive signals are combined. By adjusting the relative intensities,as with series resistors on the detectors, or amplifiers of differinggain, the system sensitivity can be optimized for the application.

The signals from the two are monitored and compared by a processor 29.Variations in light amplitude which are not related to an intrusion, andtherefor a shift in SOP, will appear on Rx1 26. Shifts in SOP will bedetected by Rx2 28. The inability to detect non-linear SOP is not assignificant as might first seem because cable handling causes dramaticchanges in SOP, with frequent motion between circular, elliptical, andlinear states.

The embodiment in FIG. 5 is a variation on this: the two opticalsignals, rather than feeding dual detectors, are joined in a coupler 30and feed a single receiver/detector 31. A significant advantage withthis arrangement is a simple one detector feeding the analysis system.The ratio of polarization sensitive to polarization insensitive can beadjusted by varying the split ratios of the two couplers 25 and 30. Thisallows significant attenuation monitoring with slight polarizationsensitivity, strong polarization sensitivity with slight attenuationsensitivity, and every combination in between.

The embodiment in FIG. 6 will detect both linear and circular polarizedlight. Light is split by a coupler or splitter 32. One leg feeds alinear polarizer 33, which allows the receiver/detector to detect linearpolarized light. The light exiting the other leg of the splitter feeds a¼ wave retarder 35 which, as in a full polarizer, converts circularpolarized light to linear polarized and vice versa. This feeds linearpolarizer 36, which allows receiver/detector 37 to detect that which wasoriginally circular polarized light. In operation, it is similar to afull polarimeter, with two primary differences:

1. The lack of the 45-degree offset 2^(nd) linear polarizer limitsresolution of linear polarized light. This causes a decrease in“intrusion gain” under some conditions.

2. This device does not measure polarization in absolute terms; ratherit detects changes in polarization, as such an absolute alignment of thesystem is not required. This greatly reduces manufacturing costs. Thusthe splitter 32 is selected such that it can be manufacturedeconomically. Preferably, the two legs of splitter 32 should be ofidentical SOP.

Note that couplers do not need to be polarization maintaining becauseonly change in SOP is important, not absolute SOP. This technique can beapplied to a full polarimeter design while neglecting calibration andalignment. In these configurations, absolute SOP measurements areinconsequential, only change in SOP is required for IDS.

Rather than use extensive DSP to calculate DOP, orientation, and angle,a system is shown in FIG. 7 for pragmatically determining polarizationchanges only.

The outputs from S1 through S3 are individually monitored in addition tobeing sent to differentiators and precision full wave rectifiers(absolute value circuits). These three rectified lines are then summedin a summing amp, the output of which is monitored.

Whenever any change in polarization properties occurs, the distributionof power between S1, S2, and S3 changes; as one increases, it is at theexpense of one or both of the others decreasing. If all three lines aredifferentiated and full wave rectified, the increase on one line willappear as a positive signal into the summing amp, as will the decreaseson the other two lines. This additive action increases sensitivity ofthe system, always positive going and of level representative of thedisturbance of the intrusion. Also, by monitoring both S0 and the threenon-processed lines S1, S2, and S3, disturbances not related tointrusion, such as a shift in laser power, could be recognized andprocessed. One possible way to analyze that condition would be, when theS0 changes, to confirm that the ratio of the S1-S3 remains the same (itspolarization “signature”) and confirm that total power changes the samedegree as S0. Because of the hardware rectification, this analysis wouldbe approximate. In implementations which do not first rectify thesignal, the above process can be performed.

The differentiation and precision rectification can be performed insoftware if desired.

Turning now to the locating system shown in FIGS. 8 to 14, thearrangement shown and described herein use the techniques describedabove.

Fundamental to the invention is the mechanics, or more simply bylaunching polarized light pulses from a light source 51 into an opticalsplitter or coupler 52. The output of the coupler is attached to themonitored fiber 54. Optical reflections caused by RayleighBackscattering and Fresnel Reflections from the fiber pass throughsplitter 52 and are fed into a polarization sensitive receiver 53. Thesignal is then processed by the processor 55: such as, but not limitedto, an A/D connected to a microprocessor.

Handling of the fiber cable causes a local mechanical disturbance to thefiber. This mechanical disturbance, while not introducing detectablemacro or micro bending losses, causes the polarization orientation tochange. This is detected by the polarimeter and reported to theprocessor. A more comprehensive view is now described.

The optical signal feeds the polarimeter 56, which converts it to thefour so-called Stokes Parameters: S0, S1, S2, and S3 as detected byreceivers Rx0-Rx3 57, 58, 59, and 60. These parameters collectivelydescribe all possible states of polarization and degrees of polarization(DOP) (Hernday 1998). This is forwarded to the processor 61 where theSOP and DOP are calculated, and the signal is filtered to eliminatenormal environmental background noise. The filtered signal is thenanalyzed for transient signatures and level changes that arecharacteristic of cable and fiber handling. At a pre-set disturbancelevel or slope change the circuit activates the alarm response.

Present art consists of the Polarization OTDR as described by Andersonand Bell (1997), which presented a characterization of the staticpolarization condition of the light as a function of distance. It didnot address intrusion, and was only intended to measure a fundamentalcharacteristic of the light within a fiber.

The invention described in this document builds upon the PolarizationOTDR by analyzing dynamic distribution of SOP throughout a fiber as anintrusion detection system. It is intended for characterizing transientSOP behavior, which was not addressed at all in prior art.

It is possible to use a single set of detection optics and electronicswhen configuring a full or partial polarimeter for applicationsincluding, but not limited to, intrusion detection in optical fiber.

In the first configuration shown in FIG. 10, an optical switch 71, ofthe simplified type described hereinbefore, selects between directmeasurement on line 72, measurement of circular polarized light on line73, and measurement of linear polarized light on line 74. This allowsone to time division multiplex (TDM) the data, using a further opticalswitch 75 scanning a fiber under test (FUT). This design allows the useof a 1×3 coupler/splitter rather than switch; potentially offering acost advantage, although with the disadvantage of several dB ofinsertion loss in the coupler/splitter. It is critical that either 71 or76, or both, be a switch as two couplers will not allow TDM. Sinceintrusions tend to be very slow occurrences, on the order of hundreds ofmilliseconds, there is ample time to average readings under eachmeasurement state.

A second configuration exists in FIG. 11, which can be chosen for costas well as other reasons. Significant to the design is the use of a timedivision switch 77 to route the signal first to a quarter wave retarder78 and then to the linear polarizer 79, which would remove the need fortwo linear polarizers. This is because the circular polarizer consistsof a quarter wave retarder followed by a linear polarizer. This designalso allows the use of a 1×2 coupler/splitter 80 rather than switch;potentially offering a cost advantage, although with the disadvantage ofseveral dB of insertion loss in the coupler/splitter.

The truth table for this configuration follows:

-   -   S0 I→A a→1    -   S1 I→B b→1    -   S2 I→C, II→B b→1

One technique for minimizing/streamlining this is to collect and storedistance data in a register 81 or other similar device such as, but notlimited to, a FIFO; but to only analyze the quasi-CW signal from theFresnel reflections in real time. This “quasi-CW” signal is comprised ofthe Fresnel reflections from the trace with a minor Rayleigh scatteringcomponent. These Fresnel reflections, on the order of 20-25 dB above thescattering are high in amplitude but low in duty cycle. They can beintegrated along with the scattering, or captured by peak detectingsample and hold (or other technique). This quasi-CW signal is analyzedfor an intrusion. When one is detected, the time dependant data in theregister 81 is analyzed for location information.

The processing required for signal analysis of an intrusion detectionsystem is not insignificant, algorithms which analyze the environmentand filter out disturbances to be ignored are highly computationallyintensive. When configuring a locating IDS, the task becomes much morecomplex. The signal analysis normally used for non-locating might needto be applied to every location in time along the vertical axis of theimaginary OTDR trace, perhaps 2000 locations or more. The CPU burden ofapplying conventional finite DSP to each of these elements is extreme.Thus the above technique of storing the data in the register until anintrusion event is detected can be used. While the intrusion event canbe most effectively detected from the Fresnel reflections, othertechniques using the other data such as data corresponding to a specificlocation in the fiber can be used to detect the intrusion event in realtime; and only when the event has been detected is the bulk of theremaining data from the register used for location. The scatteringsignal level is typically orders of magnitude lower than the FresnelReflections and the Fresnel Reflections are typically infrequent so thatthe reflections are integrated along with the scattering such that theFresnel Reflections integrate into a manageable signal and the totalintegrated signal is monitored for indication of fiber manipulation.Also the large Fresnel reflections can be sampled using a storagetechnique, this stored sample is compared to other dynamic or storedsamples and this comparison is monitored for indication of fibermanipulation.

One variation is to add a reflection at the far end of the cable, suchas a connector with a gold deposition.

It will be appreciated that the monitoring system can be used with darkfiber either which are available as spare fibers or which arespecifically dedicated as monitoring fibers. However in other cases, themonitoring system can be used with active fibers carrying data. In thiscase, if the monitor is to be used concurrently with data, a wavelengthselective reflection can be used at the remote end to increase and/orcontrol the intensity of the Fresnel reflections, such as a printedBragg Grating 90, a wave length division multiplexer (WDM) 91 and areflective connector 92, or a WDM 93 and fiber loop back 94, as shown inFIGS. 12, 13 and 14 respectively.

Turning now to the third set of embodiments shown in FIGS. 15 to 20, thearrangements are similar to those shown and described above and use manyof the same techniques. Thus it will be appreciated that each of thetechniques described can be used symmetrically. In addition, in thearrangement shown in FIGS. 15 to 20, the detections system is located atthe remote end from the signal transmission in a non-locating modesimilar to that of FIGS. 1 to 7. However, symmetrically to that of FIGS.8 to 14 the detection system can be located at the same end as thetransmission for a locating arrangement responsive to reflected andbackscattered light and may use the same techniques as described.

Thus as shown the arrangement includes a transmitter launching a lightsource of stable polarization 101 into a single mode fiber 102. At theremote or receive end the single mode fiber is connected to the input ofan optical polarizer 103. This polarizer passes light with similarlyaligned polarization, and blocks light orthogonally aligned. The outputof this polarizer is connected to an optical receiver 104. Handling ofthe fiber cable causes a local mechanical disturbance to the fiber. Thismechanical disturbance, while not introducing detectable macro or microbending losses, causes the polarization orientation to change. Thisresults in a change in the optical power at the output port 105 whichfeeds the receiver. The resultant optical signal is proportional inamplitude to the disturbing forces.

In the case of active fiber monitoring, where live traffic is carried onthe monitored fiber, as shown in FIG. 16, the optical signal from thesource 106 is split by an optical coupler 107. The main portion of thesignal can be brought back out to an optical connector 108 and be madeavailable for the communication or data receiver. The sampled output 109feeds the polarizer 110, which feeds the receiver 111. The signal can bedigitized and forwarded to the processor 112 where the signal isfiltered to eliminate normal environmental background noise. Thefiltered signal is then analyzed for transient signatures and levelchanges that are characteristic of cable and fiber handling. At apre-set disturbance level the circuit activates the alarm response.

An enhanced variation of the detection scheme is shown in FIG. 17. Theincoming optical signal from the fiber 113 is connected to the input ofa 2×1 coupler 114 where a portion of the light is sampled. One output ofthe coupler 115 is then connected to the input port of a polarizer 116as above. The coupler maintains polarization information and it is usedto sample a portion of the total optical signal. The other output of thecoupler 117 is connected to a second receiver 118 where the absolutethroughput power is calculated from the fixed ratio sample. Thisestablishes an absolute power baseline that is compared to thepolarization detection sampling. The processor then compares theresponse in the two channels and is able to calculate any power changeas well as changes in polarization. This comparison can be performed inthe digital domain including use of equipment such as, but not limitedto a computer, or the analog domain using circuitry such as, but notlimited to, a differential amplifier. This provides more information onfiber disturbances as a significant change in both channels couldindicate a problem with the laser or fiber path while a transient andsteady state change in the polarization only would provide a strongindication of an intrusion attempt.

The techniques described above can be combined, as illustrated in FIG.18. The tap coupler of FIG. 16 and the dual receiver of FIG. 17 areimplemented.

In FIGS. 15, 16, 17, and 18, the polarizer 103, 110, and 116 can bereplaced by a polarization controlling device 120, as shown in FIG. 19.Under feedback control, the base polarization state can be adjusted toany level within the extinction ratio of the polarizer to optimize theefficiency and sensitivity of the measurement.

An embodiment shown in FIG. 20 consists of launching a light source ofstable polarization 121 into a single mode fiber 122. At the remote orreceive end the single mode fiber is connected to the input of anoptical splitter or coupler 123, typically of a non-symmetrical splitratio. One output of this coupler, typically the larger couplingpercentage leg, feeds the optical connector 124 to the end receiver. Theother leg of coupler feeds an optical splitter or coupler 125. Typicallythis would be a 50:50 coupler. One leg of this coupler feeds an opticalreceiver 126. The other coupler leg feeds a polarization controller 127,which alters the state of polarization (SOP) of the exiting light. Thisfeeds the input to an optical coupler or splitter 128, typically of a50:50 split ratio. The two output legs of this coupler feeds a pair ofoptical polarizers 129 and 130, whose SOP is aligned orthogonal to eachother. These feed a pair of optical receivers 131 and 132. Aprocessor/controller 133, such as a combination of ND converters and CPUmonitors the outputs of the three receivers 126, 131, and 132, andadjusts the controller 27 accordingly.

In order to maximize detection sensitivity, or “intrusion gain”, theoptics must be aligned such that the signal at Rx3 132 is at a minimum;i.e. Pol 2 130 perfectly orthogonal to the light. This signal is,however, very low in magnitude and difficult to measure. One way ofinsuring this alignment is to align Polarization Controller 127 for amaximum signal at Rx2 131.

The polarization controller 127 is a device that can convert any SOPinto any other SOP. Using this, a transmitter laser can be easilyconverted into a more readily managed linear polarization. The Processor133 adjusts SOP by monitoring Rx2 131 for a maximum signal. When thisoccurs, the SOP is properly aligned and linear. Rx3 132 then monitorsfor intrusions.

Additionally, the ratio of signals at Rx2 131 to Rx1 126 is anindication of the “tuning” of polarization alignment. When Rx2 drops inpower while Rx1 remains constant, alignment issues or an intrusion areoccurring. If they both change in power, an attenuation event isoccurring, such as laser power fluctuation or a failing connector.

Thus in FIG. 20 a polarized light source launched into a length ofsingle mode fiber. The single mode fiber is connected to the opticalcoupler which splits the signal: the majority going to the system datareceiver (if an active fiber unit), a portion sampled to the measurementsystem. This sampled portion goes to another splitter, one leg of whichmonitors power, the other leg is monitored for intrusion. This intrusionleg feeds a polarization controller, which both takes whatever stableSOP of the light and converts it to linearly polarized, and aligns itwith the orientation of polarimeter Pol 1. Pol 1 feeds Rx2 and allows astrong signal for both closing the control loop on the polarizationcontroller, as well as monitoring, with Rx1, fluctuations in absolute(non-intrusion dependant) power. Pol 2 feeds Rx3; and, being alignedperpendicular to the SOP of the light, is optimized for intrusionsensitivity.

In summation: Rx1 measures absolute optical power, Rx2 monitors maximumpolarized power, and Rx3 monitors intrusion. The combination of Rx1 andRx2 monitor systematic stabilities not related to intrusion. Thecombination Rx1 and Rx3 detect actual intrusions.

Since various modifications can be made in my invention as herein abovedescribed, and many apparently widely different embodiments of same madewithin the spirit and scope of the claims without department from suchspirit and scope, it is intended that all matter contained in theaccompanying specification shall be interpreted as illustrative only andnot in a limiting sense.

1. A method for use with a telecommunications optical cable having a plurality of optical fibers, the method comprising: transmitting into at least one of the optical fibers of the cable transmitted optical signals so as to generate received signals from light which has been transmitted along said at least one of the optical fibers in response to said transmitted optical signals; causing a comparison of characteristics of at least some of the received signals relative to characteristics obtained from previously received ones of the received signals to detect changes in the characteristics of the received signals relative to the previously received signals; filtering the received signals to filter background disturbances from the received optical signals by learning those signals which arise from background disturbances and filtering those signals out to generate filtered signals; from the comparison of the filtered signals, determining any changes in the characteristics.
 2. The method according to claim 1 including generating an alarm indicative of an intrusion from any changes in characteristics which are indicative of an intrusion.
 3. The method according to claim 1 wherein the characteristics are selected to that they are indicative of a manipulation of the optical fiber which causes movement of a portion thereof along the length thereof.
 4. The method according to claim 1 wherein the filtering is carried out by adaptive filtering.
 5. The method according to claim 1 wherein the characteristics of the light signals relate to polarization of the light signals.
 6. The method according to claim 1 wherein said at least one of the optical fibers comprises a single mode fiber.
 7. The method according to claim 1 wherein the received signals are at a remote end from a transmitted end.
 8. The method according to claim 1 wherein the changes in the characteristics are selected so that they are indicative of manipulation of the optical fiber and the selected characteristics act to identify intrusion attempts while the attack is still at the outer layer of the cable structure.
 9. The method according to claim 1 wherein the characteristics of the received signals relate to signal degradation and disturbances.
 10. The method according to claim 1 wherein the transmitted optical signals are provided from a polarized light source.
 11. The method according to claim 1 wherein said at least one of the optical fibers is a dark fiber.
 12. The method according to claim 1 wherein said at least one of the optical fibers is an active fiber carrying data. 